1. Field of the Invention
The present invention relates to separators for alkaline storage batteries and alkaline storage batteries using the same.
2. Description of the Related Art
In recent years, alkaline storage batteries have been used as power sources for portable information equipment such as cellular phones and notebook computers, electric cars, or hybrid vehicles, and there has been a demand for high performance, in particular long life of alkaline storage batteries. In order to achieve long life of batteries, it is important to ensure the liquid retention properties of a separator, in particular, when pressure is applied to the separator by an electrode plate. In order to improve the liquid retention properties at the time of applying pressure, it is necessary to increase the hydrophilicity and the surface area of the separator.
Conventionally, a separator made of a polyolefin resin that has been subjected to a sulfonation treatment has been used as the separator for alkaline storage batteries. The separator that has been subjected to a sulfonation treatment has excellent properties of suppressing self-discharge. In order to increase the hydrophilicity of this sulfonated separator, it is necessary to increase the degree of sulfonation.
However, when the sulfonation is excessive, the strength of the separator decreases, so that a short-circuit occurs easily.
On the other hand, in order to increase the surface area of a separator, the mass per unit area of a separator is increased, or fine fibers are used. However, these approaches have a problem in that the air permeability of the separator is decreased and the internal pressure is increased. For example, gas produced in a positive electrode at the time of overcharge permeates through the separator and is consumed in a negative electrode, but if the air permeability of the separator is low, this reaction is suppressed so that the internal pressure of the battery is increased. Then, if the internal pressure of the battery is increased to be higher than the pressure of the safety valve, gas inside the battery is released to the outside so that the life of the battery is reduced.